Blends of polyolefin (e.g., polypropylene) and resin modifier (e.g., hydrogenated petroleum resin) have commercial importance. For example, these materials are often blended and formed into films having desirable stiffness, clarity, heat sealability and/or barrier properties. These films are often produced using film extruders, which typically include a single-screw extruder for processing and conveying the materials. In order to effectively use a single-screw extruder for this purpose; however, prefabricated blends of the polyolefin and hydrocarbon resin had to be prepared. In other words, the very large difference in softening and melting temperatures between polyolefin and hydrocarbon resin, as well as the rheological differences in their properties, has proven to be an insurmountable barrier to directly adding these materials to a single-screw extruder for purposes of mixing and processing the materials in a single step. Of the many problems experienced are poor dispersion of the modifier within the polyolefin, extrusion instability, surging, and low output rate.
In order to overcome the problems in the prior art, U.S. Pat. No. 5,213,744 teaches a process whereby a concentrate or masterbatch of polyolefin and modifier (which is advantageously in the form of pellets) is prepared in a twin-screw extruder and the concentrate is subsequently charged to a single-screw extruder for processing and conveying in fabrication processes such as film forming processes. The formation of the concentrate advantageously allowed for the introduction of additional polyolefin and the successful blending a processing of the polyolefin together with the concentrate within the single-screw extruder.
In addition to forming a concentrate as taught by the US'744, the prior art clearly suggests that the problems associated with blending polyolefin and modifier can be overcome by using a twin-screw extruder. In other words, if a twin-screw extruder were used during the fabrication process (e.g., film forming process), then the blending, processing and conveying of the polyolefin and modifier can be achieved without the need for the concentrate. While apparent, this solution is not practical because of the significant capital costs associated with a twin-screw extruder.
Since production of the concentrate of polyolefin and modifier can be time consuming, energy consuming, and economically disadvantageous, there remains a need to overcome problems associated with blending polyolefin and resin modifier within a single-screw extruder, which solution will allow one-step fabrication processes, such as film-forming processes.